1. Field Of The Invention
The present invention relates to composite materials technology, and more specifically to a relatively light-weight, inexpensive, durable, high performance structural laminate composite material for use to 1000.degree. F., and above, which can advantageously be used in high temperature environments. More particularly, the preferred embodiment of the present invention relates to a graphite-fiber/phenolic-resin composite material which retains relatively high strength and modulus of elasticity at temperatures as high as 1,000.degree. F. (538.degree. C.). The material costs only 5 to 20 percent as much as refractory materials do. The fabrication of the composite includes a curing process in which the application of full autoclave pressure is delayed until after the phenolic resin gels. This modified curing process allows moisture to escape, so that when the composite is subsequently heated in service, there will be much less expansion of absorbed moisture and thus much less of a tendency toward delamination. In contrast, internal pressure caused by the expansion of moisture absorbed in other prior art composite materials like prior art graphite/epoxies and prior art graphite/polyimides causes delamination at temperatures in the range of 500 to 700.degree. F. (260 to 370.degree. C.).
2. General Background
At the request of NASA/MSFC, Martin Marietta Manned Space Systems has performed an extensive development/verification activity for a composite nose cone for the external tank (ET). At the time of the initiation of this effort, there was no materials technology available to provide a nose cone which could withstand the high heating and structural loading of the ET nose cone without (a) requiring the use of secondary heat shield materials, (b) increasing the weight of the existing nose cone, and (c) significantly increasing the cost over the existing nose cone cost. There were high temperature polymeric composite materials available, however, none met all requirements. Carbon/phenolic laminates have been proven in rocket nozzle applications to be able to withstand extreme heating conditions; however, these materials did not possess the specific strength and stiffness required for a weight-effective structure. Also, recent data shows that the materials on the market today have the potential to "ply lift," or delaminate due to internal pressure caused by absorbed moisture, at about 500.degree. F. Graphite/polyimide laminates showed promising mechanical properties, but suffered from the moisture-induced delamination problem (also known as "thermal shock") at temperatures below 700.degree. F. in laminates of the thickness required for a composite nose cone. Other technologies such as ceramic matrix composites and carbon/carbon were considered too expensive for this application. Therefore, a program was initiated to develop laminate material which could meet all requirements.
U.S. Pat. No. 3,724,386 for "Ablative Nose Tips and Method for their Manufacture" discloses in Example II heating graphite yarn impregnated with phenolic resin slowly to 160.degree. F. to slowly evaporate solvent from the resin (see column 8, lines 16-18).
U.S. Pat. Nos. 4,100,322 and 4,215,161 for "Fiber-Resin-Carbon Composites and Method of Fabrication" disclose impregnating graphite yarn with phenolic resin under vacuum and a temperature of about 150.degree. F. until the solvent has gone and the resin gels, then further heating the composite to cure it. However, the solvent stripping process was interrupted twice and each time pressure of 200 psig was applied to the composite material. It is then subjected to pyrolysis, and then pores of the composite are impregnated with phenolic resin. After this, the phenolic resin is cured at about 350.degree. F. The resulting structure is said to be graphite/carbon/phenolic composite, and its porosity is disclosed to be 4%. A carbon/carbon/phenolic composite described therein is said to have a porosity of 5.8%.
U.S. Pat. No. 4,659,624 for "Hybrid and Unidirectional Carbon-Carbon Fiber Reinforced Laminate Composites" discloses a method similar to the method disclosed in U.S. Pat. Nos. 4,100,322 and 4,215,161 (and with similar materials), but one in which more resin is added and pyrolized up to 5 times. This patent points out at column 2, line 50 through column 3, line 2 that it is important to properly initially cure laminate materials to provide interconnecting pores which allow the escape of gases formed during post-cure pyrolysis.
U.S. Pat. No. 4,957,801 for "Advance Composites with Thermoplastic Particles at the Interface Between Layers" discloses a resin-impregnated fiber layer with outer layers of resin thereon. The fiber can comprise, for example, graphite.
U.S. Pat. No. 5,288,547 for "Toughened Resins and Composites" discloses a composite in which a porous membrane film of thermoplastic material is sandwiched between two layers of resin-impregnated fibers, and then the composite is cured in an autoclave, for example. The resin can be, for example, phenolic resin.
U.S. Pat. No. 5,359,850 for "Self Venting Carbon or Graphite Phenolic Ablatives" discloses a resin-impregnated reinforcing cloth made of, for example, graphite fibers with degradable fibers interwoven therewith. The degradable fibers are chosen such that they degrade at a temperature of about 400.degree. F. to 500.degree. F. so that they will provide passageways for the gaseous decomposition products produced as the resin matrix approaches the char temperature. In this patent, foreign material is introduced to create porosity. The fabric weave is altered by introducing a low-temperature degradable thread which may not assure fabric strength properties. The porosity which is created by this process is uniform. There is a definite pattern when the foreign material is replaced by voids. It is believed that the addition of these special degradable fibers will add to the cost of the material. Further, it is believed that in some cases the degradable fibers might not burn away before the plies blow apart.
U.S. Pat. No. 5,360,500 for "Method of Producing Light-Weight High-Strength Stiff Panes" discloses a panel made by a pair of surface members separated and supported by an internal core in which spaces or interconnected pores provide vents to an edge of the panel so that gas can flow through the vents during a pyrolysis process. The vents are on the order of 10 mm in diameter.
None of these patents discloses a composite material with a weight, thickness, structural performance, and pore structure as advantageous for use in a nose cone of the external tank of the space shuttle, or other high temperature structural applications, as the material of the present invention.